In 1975-76, Diffie and Martin Hellman criticized the NBS proposed Data Encryption Standard, largely because its 56-bit key length was too short to prevent Brute-force attack. An audio recording survives of their review of DES at Stanford in 1976 with Dennis Branstad of NBS and representatives of the National Security Agency.[6] Their concern was well-founded: subsequent history has shown not only that NSA actively intervened with IBM and NBS to shorten the key size, but also that the short key size enabled exactly the kind of massively parallel key crackers that Hellman and Diffie sketched out. When these were ultimately built outside the classified world, they made it clear that DES was insecure and obsolete. In 2012, a $10,000 commercially available machine can recover a DES key in days.

Diffie was Manager of Secure Systems Research for Northern Telecom, where he designed the key management architecture for the PDSO security system for X.25 networks.[5]

In 1991 he joined Sun Microsystems Laboratories (in Menlo Park, California) as a Distinguished Engineer, working primarily on public policy aspects of cryptography. Diffie remained with Sun, serving as its Chief Security Officer and as a Vice President until November 2009. He is also a Sun Fellow.[7]

In the early 1970s, Diffie worked with Martin Hellman to develop the fundamental ideas of dual-key, or public key, cryptography. They published their results in 1976—solving one of the fundamental problems of cryptography, key distribution—and essentially broke the monopoly that had previously existed where government entities controlled cryptographic technology and the terms on which other individuals could have access to it. "From the moment Diffie and Hellman published their findings..., the National Security Agency's crypto monopoly was effectively terminated. ... Every company, every citizen now had routine access to the sorts of cryptographic technology that not many years ago ranked alongside the atom bomb as a source of power."[4] The solution has become known as Diffie–Hellman key exchange.

^ abcdLevy, Stephen (1994-07-12). "Battle of the Clipper Chip". New York Times Magazine. pp. 44–51, plus cover photo of Diffie. Whitfield Diffie's amazing breakthrough could guarantee computer privacy. But the Government, fearing crime and terror, wants to co-opt his magic key and listen in. ... High-tech has created a huge privacy gap. But miraculously, a fix has emerged: cheap, easy-to-use-, virtually unbreakable encryption. Cryptography is the silver bullet by which we can hope to reclaim our privacy. ... a remarkable discovery made almost 20 years ago, a breakthrough that combined with the obscure field of cryptography into the mainstream of communications policy. It began with Whitfield Diffie, a young computer scientist and cryptographer. He did not work for the government. ... He had been bitten by the cryptography bug at age 10 when his father, a professor, brought home the entire crypto shelf of the City College Library in New York. ... [Diffie] was always concerned about individuals, an individual's privacy as opposed to Government secrecy. ... Diffie, now 50, is still committed to those beliefs. ... [Diffie] and Martin E. Hellman, an electrical engineering professor at Stanford University, created a crypto revolution. ... Diffie was dissatisfied with the security [on computer systems] ... in the 1960s [because] a system manager had access to all passwords. ... A perfect system would eliminate the need for a trusted third party. ... led Diffie to think about a more general problem in cryptography: key management. ... When Diffie moved to Stanford University in 1969, he foresaw the rise of home computer terminals [and pondered] how to use them to make transactions. ... in the mid-1970s, Diffie and Hellman achieved a stunning breakthrough that changed cryptography forever. They split the cryptographic key. In their system, every user has two keys, a public one and a private one, that are unique to their owner. Whatever is scrambled by one key can be unscrambled by the other. ... It was an amazing solution, but even more remarkable was that this split-key system solved both of Diffie's problems, the desire to shield communications from eavesdroppers and also to provide a secure electronic identification for contracts and financial transactions done by computer. It provided the identification by the use of 'digital signatures' that verify the sender much the same way that a real signature validates a check or contract. ... From the moment Diffie and Hellman published their findings in 1976, the National Security Agency's crypto monopoly was effectively terminated. ... Every company, every citizen now had routine access to the sorts of cryptographic technology that not many years ago ranked alongside the atom bomb as a source of power.'|accessdate= requires |url= (help)

^ ab"The People at Oracle Labs". Bio. Oracle Corporation. n.d. Retrieved 2011-01-28. Whitfield Diffie, Chief Security Officer of Sun Microsystems, is Vice President and Sun Fellow and has been at Sun since 1991. As Chief Security Officer, Diffie is the chief exponent of Sun's security vision and responsible for developing Sun's strategy to achieve that vision.

^"Cryptography Legend Whit Diffie Joins the ICANN Team"(PDF). ICANN News Release. ICANN. May 14, 2010. Retrieved 2011-01-28. Globally recognized as a leader in public-key cryptography, encryption and network security, Diffie has a long and distinguished career as a leading force for innovative thought. He brings extensive experience in the design, development and implementation of security methods for networks. ... Prior to coming to ICANN, Diffie served as Vice President, Fellow, and Chief Security Officer with Sun Microsystems, at which he had worked from 1991 to 2009. At Sun, Diffie focused on the most fundamental security problems facing modern communications and computing with emphasis on public policy as well as technology. Prior to joining Sun, Diffie was Manager of Secure Systems Research for Northern Telecom, where he played a key role in the design of Northern's first packet security product and in developing the group that was later to become Entrust.